As is known, many food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated sheet packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may comprise a layer of fibrous material, e.g. paper, or of mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH), which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating; and the web of packaging material so sterilized is maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a vertical tube.
Packaging machines comprise a forming unit, wherein the tube is filled continuously downwards with the sterilized or sterile-processed food product, and is sealed and then cut along equally spaced cross sections to form pillow packs, which are then fed to a folding unit to form the finished, e.g. substantially parallelepiped-shaped packages.
More specifically, the pillow packs substantially comprise a parallelepiped-shaped main portion; and a top end portion and a bottom end portion, opposite to each other and projecting laterally on opposite sides of the main portion and defining respective triangular end flaps to be folded onto the main portion.
A longitudinal sealing strip, formed when sealing the packaging material to form the vertical tube, extends along the pillow packs; and the top end portion and bottom end portion of each pillow pack have respective transverse sealing strips perpendicular to the longitudinal sealing strip and defining respective end flaps projecting from the top and bottom of the pack.
The top end portion and the bottom end portion of each pillow pack taper towards the main portion from the respective end flaps, and are pressed towards each other by a folding unit of the packaging machine to form flat opposite end walls of the pack, while at the same time folding the end flaps onto respective walls of the main portion.
Folding units are known, for example from EP-1726526, substantially comprising a chain conveyor for feeding packs continuously along a predominantly straight horizontal forming path from a supply station to an output station, and a plurality of folding devices which cooperate cyclically with each pack along the forming path to flatten the respective top end portion and bottom and portion of the pack and so fold the respective end flaps onto the end portions.
The folding unit comprises heating means arranged for heating the packs and melting the plastic material forming the outer plastic layer of the packs at the top end portion and bottom end portion.
One of the above-mentioned folding devices comprises a pressing unit arranged above the chain conveyor so as to interact with the packages. The pressing unit presses the end flaps onto the respective walls of the main body and, as the plastic material sets, the end flaps are sealed to the walls of the main body.
The pressing unit comprises a belt having the shape of a loop and provided with a lower branch arranged to interact with the packs to press the end flaps. The pressing unit comprises a driving element which drives the belt so that the rotation of the belt is synchronized with the movement of the chain conveyor.
The pressing unit further comprises a plate, facing toward the chain conveyor, on which the lower branch of the belt slides when the belt is rotated. In practice, the plate prevents the belt from moving away from the packages during mutual interaction, so allowing the belt to exert the required pressure on the end flaps.
The belt further comprises a plurality of bulges, projecting from the belt towards the chain conveyor and having a shape substantially matching with the end zone of the pack they have to push. The bulges are spaced along the belt so that each bulge interacts with a corresponding pack being advanced by the chain conveyor.
Though reliable and efficient, the folding units described still leave room for further improvement.
A drawback of the folding unit is that the pressing unit has to be equipped with a cooling system for dissipating the heat produced by the friction, since the belt slides on the plate at a high speed. The metal body forming the plate, therefore, has to be provided with channels, or pipes, for a cooling fluid.
Another drawback of the folding unit is that the belt—due to the interaction with the plate—becomes worn after a certain number of working hours and has to be replaced.
The above-mentioned drawbacks are magnified in case of packaging machines having an increased output rate, which means correspondingly increased speeds of the chain conveyor and of the belt of the pressing unit.